API endpoint PUT /projects/:id/merge_requests/:merge_request_iid/approvers executes more than 100 SQL queries

This controller executes more than 100 SQL queries, the maximum number of queries allowed as per MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16466.

The main cause is due to the development of #1979 (closed), where data migration is needed between the old and new schema. An after_commit background migration hook is added whenever Approver or ApproverGroup is created or destroyed, making updates on corresponding approval_rule.

This end point will be deprecated. After its removal in 12.0, we can close this issue.